Self-calibrating electronic distance measurement instrument

ABSTRACT

A combined satellite positioning and electro-optical total station system includes a reference oscillator that provides local oscillator signals for a satellite navigation receiver and a precision frequency source for use by an electronic distance meter. When the satellite navigation receiver is locked onto and tracking orbiting navigation satellites, the highly precise cesium-rubidium clocks in the navigation satellite system can be used as standards to control the reference oscillator in the combined satellite positioning and electro-optical total station system. Baseline measurements made by the electronic distance meter are therefore not subject to mis-calibrations and drift as long as the satellite navigation receiver is locked onto and tracking the orbiting navigation satellites.

RELATED APPLICATION

This Application is a divisional of U.S. patent application Ser. No.09/163,286, filed Sep. 28, 1998, which is a continuation-in-part (CIP)of an earlier filed U.S. patent application Ser. No. 09/122,265, filedJul. 24, 1998 now abandoned, by both of the present inventors, NicholasCharles Talbot and Michael V. McCusker.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to surveying instruments, andmore particularly to devices and methods for using reference signalsfrom a satellite navigation receiver to automatically and preciselycalibrate electronic distance measurement instruments, and forservo-driving the telescopes in electro-optical total stations. Suchcalibration specifically includes hardware techniques for aligning areference clock or oscillator, and/or software techniques for measuringlocal clock and frequency offsets and then subtracting such offsets outin the final calculations.

2. Description of the Prior Art

Electronic distance measurement (EDM) equipment became commerciallyavailable after World War-II and has since become very important to thesurveying, navigation and scientific communities. Since the introductionof EDM, the instrument size and power consumption have been reduced, andthe precision and speed of measurement have been improved. Because theminiaturization of EDM equipment became possible, it made good sense tomount EDM's on theodolites which have telescopes that can preciselysight a horizontal and vertical angle to a target. Such combinations areelectro-optical hybrids called “total stations.”

Combination electronic theodolite and EDM instruments allow surveyors tofind the “space vector” from the instrument to a distant target. When atotal station is connected to an electronic data recorder, fieldinformation can be quickly gathered and used to generate maps and plansin the office.

Flexible tapes, leveling staves, electro-optical distance meters, andother surveying equipment are calibrated to a legal standard andcalibration certificates are issued, e.g., a “Regulation 80Certificate,” as is issued in Western Australia. Such calibration isespecially important where a legal purpose is in mind, e.g., aninspection to enforce a law or to be used as evidence in a court action.A flexible tape calibration laboratory in Midland is registered by theNational Standards Commission of Australia for calibration of 1-100meter lengths.

There are two certified baselines in Western Australia against which EDMinstruments can be calibrated. The aim of EDM calibration is to ensurethat it measures in accordance with the internationally recognizeddefinition of length, as set forth by the Conference Generale des Poidset Measures (CGPM—the General Conference on Weights and Measures). Othergovernments in the world provide similar baselines and certificationopportunities. When a Regulation 80 Certificate is required for thepurpose of legal traceability to the Australian Standard for length, theEDM instrument is submitted to the Surveyor General for calibration. TheDirector of the Mapping & Survey Division is the verifying authority forlength and is appointed by the National Standards Commission. TheSurveyor General now provides a software application program, calledBASELINE, to assist surveyors with their regular calibrations of EDMinstruments.

The accuracy of electronic distance measurement equipment is derivedfrom an internal reference frequency source, e.g., a crystal oscillator.But such crystal oscillators can drift over time and with age. Exposureto extreme environments can also upset delicate calibrations of thereference frequency source, both short term and long term. Therefore,EDM equipment should be regularly calibrated by using it to measure aknown length.

Long-range electronic distance meters, e.g., ranges over fivekilometers, typically use microwave signals for measurement. Short rangeelectronic distance meters often use infrared light. See, Rueger, J. M.,Electronic Distance Measurement—An Introduction, Springer Verlag,Berlin, third edition, 1990. Both the long-range and short-range EDM'suse pulse or phase comparison methods to determine the distance betweeninstrument and a remote target. However, the phase comparison method ismore commonly used for survey instruments.

The pulse technique is based on timing the signal travel time to andfrom a distant reflector. The velocity of the signal is assumed to beknown. For phase comparison, the phase difference of signals is observedat several frequencies. The unambiguous distance between the target andthe instrument is resolved using phase difference observations. But inall cases, the basis for measurement precision depends on the accuracyof the stand-alone reference frequency source.

One of the present inventors, Nicholas C. Talbot, described a combinedsatellite positioning/electro-optical total station system in U.S. Pat.No. 5,471,218, issued Nov. 28, 1995. One candidate satellite positioningsystem that can be used effectively is the Global Positioning System(GPS) operated by the United States. Such patent is incorporated hereinby reference.

The combined satellite positioning/electro-optical total station systemallows rapid instrument orientation and positioning in the field.Another integrated surveying system that combines electro-opticalinstrumentation with a satellite position measuring system is describedby Ingensand, et al., in U.S. Pat. No. 5,233,357.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to provide a combinedsatellite positioning and electro-optical total station system in whichthe electronic distance measurement is automatically and preciselycalibrated.

It is another object of the present invention to provide a combinedsatellite positioning and electro-optical total station system thatavoids duplicating components between its satellite positioning portionand its electro-optical total station portion.

Briefly, a combined satellite positioning and electro-optical totalstation system embodiment of the present invention includes a referenceoscillator that provides local oscillator signals for a satellitenavigation receiver and a precision frequency source for use by anelectronic distance meter. When the satellite navigation receiver islocked onto and tracking orbiting navigation satellites, the highlyprecise cesium-rubidium clocks in the navigation satellite system can beused as standards to control the reference oscillator in the combinedsatellite positioning and electro-optical total station system. Baselinemeasurements made by the electronic distance meter are therefore notsubject to mis-calibrations and drift as long as the satellitenavigation receiver is locked onto and tracking the orbiting navigationsatellites.

An advantage of the present invention is that a combined satellitepositioning and electro-optical total station system is provided thatincludes an electronic distance meter that remains automaticallycalibrated.

Another advantage of the present invention is that a combined satellitepositioning and electro-optical total station system is provided that isless expensive to manufacture and maintain than the separate instrumentsit replaces.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment thatis illustrated in the drawing figure.

IN THE DRAWINGS

FIG. 1 is a functional block diagram of combined satellite positioningand electro-optical total station system embodiment of the presentinvention;

FIG. 2 represents a plot of short-term oscillator drift and the effectof the present invention to correct long-term oscillator drift;

FIG. 3 is a functional block diagram of a total station which uses anexternal reference oscillator that is stabilized by a timing signalobtained from a GPS receiver;

FIG. 4 is a functional block diagram of a 10 MHz reference oscillator ina generic product that is locally stabilized or disciplined by a GPSreceiver with zero-crossing comparisons at one pulse per second;

FIG. 5 is a functional block diagram of a 10 MHz reference oscillator ina generic product that is remotely stabilized or disciplined by radiotransmissions it receives from either a GPS receiver or governmenttime-standard broadcasts such as from WWV;

FIG. 6 is a schematic diagram of a GPS receiver useful in theconfigurations shown in FIGS. 1-3; and

FIG. 7 is a functional block diagram of a combined satellite positioningand electro-optical total station system embodiment of the presentinvention with software correction.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a combined satellite positioning and electro-opticaltotal station system embodiment of the present invention, referred toherein by the general reference numeral 10. A global positioning system(GPS) part of the system 10 includes a microwave patch antenna 12 forreceiving L-band transmissions from orbiting GPS satellites, apreamplifier and downconverter 14, a code mixer 16, an in-phase carriermixer 18, a quadrature phase carrier mixer 20. The carrier mixer outputsare each sent to low pass filters 22 and 24. A numerically controlledoscillator (NCO) 26 is driven by a bi-phase locked loop filter 28 and amultiplier 30. The NCO 26 produces a corrected frequency output thattracks the GPS-satellite carrier being tracked plus any Doppler effects.

The low pass filter 22 produces a fifty Hertz navigation message that isinput to a navigation computer 32. An adder 34 combines a squaredin-phase signal (I{circumflex over ( )}2) and a squared quadrature-phasesignal (Q{circumflex over ( )}2) to produce a signal-power signal(I{circumflex over ( )}2+Q{circumflex over ( )}2) 36 that isproportional in magnitude to the despreading code correlation. TheI{circumflex over ( )}2+Q{circumflex over ( )}2 signal 36 is used tocontrol the code-phase of a PRN-code generator 38. A GPS-masterreference oscillator 39 receives correction signals from the navigationcomputer 32 that maintain the satellite tracking. A precision referencefrequency is then made available to drive a clock 40 and thedownconverter 14. A buffer driver 41 allows the reference frequency tobe brought external from the GPS portion and isolates the referenceoscillator from external load variations.

FIG. 2 represents a plot of short-term oscillator drift and the effectof the present invention to correct long-term oscillator drift.

Referring again to FIG. 1, code measurement, time measurement, and thenavigation message are used by the navigation computer 32 to compute thecurrent three-dimensional position of the system 10. The GPS systemtime, e.g., in Universal Time Coordinated (UTC), is also determined bythe navigation computer 32. Such UTC is typically accurate in absoluteterms to better than one hundred nanoseconds. It is better than that ona relative basis, over a short term.

Once the location of the instrument station is determined either fromGPS or other means, a minimum of only one satellite is required tocalibrate the time base of the instrument.

An electronic distance meter (EDM) part of the system 10 includes aphase comparator and charge pump 42 that servo controls a slaveoscillator 43. When the GPS navigation receiver part is tracking enoughsatellites to obtain a position fix, a highly accurate estimate of timeand local oscillator frequency is available and used to precisely fixthe operating frequency of oscillator 43. Inexpensive crystaloscillators can be used throughout and for the local oscillator in theGPS receiver, and their absolute frequency accuracy is relativelyunimportant because once signal lock is obtained with the GPSsatellites, phase locked loops can be used to establish a precisionfrequency reference that is almost as accurate as the cesium-rubidiumclocks in the GPS system.

An EDM phase measurement subsystem 44 is connected to a transmitter 46that sends an out-bound signal 47 through a telescope 48 to a distanttarget 50. The target 50 may include a prism corner-cube reflector, oractive repeater for microwave EDM, to return an in-bound signal 51. Thesignals 47 and 51 may be infrared or other laser light, or microwavesignals. The EDM phase measurement subsystem 44 can conduct either pulsetime-of-flight or carrier phase measurements to determine theline-of-sight distance to the target 50. Conventional methods andequipment can be used to do this. A target range measurement 54 isoutput that can be presented on a local display, recordedelectronically, or transmitted to a user that is at the target and ismoving the target around to mark a particular range from the system 10location.

A theodolite part of the system 10 includes the telescope 48 mounted toan angle measurement instrument 56 connected to a servo actuator 58. Atheodolite measurement 60 includes an elevation and azimuth output thatcan be presented on a local display, recorded electronically, ortransmitted to a user that is at the target 50 and is moving the targetaround to mark a particular vector angle from the system 10 location. Aspace vector to target signal 62 is computed by the navigation computer32 from a target position seed input 64.

The navigation computer 32 is able to compute the current position ofthe system 10 and outputs this as a position estimate 66. From thisposition estimate, it is possible to determine the altitude and azimuthvector to the target 50. The space vector to target signal 62 commandsthe servo 58 to move the telescope 48 so that it is roughly pointed atthe target 50. A conventional search and tracking mechanism can then beused to find and keep the target 50 locked in. For example, theGeodimeter SYSTEM-500 is a commercially marketed system that is aservo-driven survey instrument in an automatically pointedelectro-optical total station. The target location seed can be computedusing differential satellite position calculations relative to the EDMreference station.

FIG. 3 illustrates a system 70 in which a total station 72 inputs a10.00 MHz precision reference oscillator 74 that is stabilized by atiming signal 75 derived from a GPS receiver 76. For example, GPSreceivers marketed by Trimble Navigation Limited (Sunnyvale, Calif.)outputs a utility one-pulse-per-second (1PPS) that can be used by aphase comparison and frequency control circuit 78 to make minorcorrections in the operating frequency of oscillator 74. Such referenceoscillator may be a voltage-controlled oscillator (VCO) or a numericcontrolled oscillator (NCO) type. For the VCO type, the control signalfrom circuit 78 is a variable analog voltage or current. For the NCOtype, the control signal from circuit 78 is a digital value.

FIG. 4 shows a precision reference system 80 in which a 10.00 MHzreference oscillator 81 is a generic product that is stabilized ordisciplined by zero-crossing comparisons at one pulse per second. Adivider 82 is used to reduce the output of the oscillator 81 to 1.00 Hz.A local GPS receiver source 83 provides a reference 1.00 Hz signal thatis exceedingly precise and stable because it is derived from the atomicclocks used in the GPS system time standards. A phase comparator 84provides an error signal 85 that is applied to an integrating filter 86that drives the static phase error to zero for synchronization. Acontrol signal 87 is returned via a buffer 88 to the oscillator 81. Theoverall effect is to reduce the accumulation of errors over the longterm to an average of zero, as in FIG. 2.

FIG. 5 shows an alternative embodiment of a precision reference system90 in which a 10.00 MHz reference oscillator 91 within an otherwisestandard commercial product is stabilized or disciplined, e.g., withone-pulse-per-second signals. A divider 92 reduces the 10.00 MHz outputof the oscillator 91 all the way down to 1.00 Hz. A radio receiver 93 istuned to a 1.00 Hz remotely transmitted signal that is exceedinglyprecise and stable. A phase comparator 94 provides an error signal 95that is applied to an integrating filter 96 that drives the static phaseerror to zero for synchronization. A control signal 97 is returned toclosed-loop lock in the frequency of operation of oscillator 91. A GPSreceiver and radio transmitter combination 98 or a governmenttime-standard broadcast transmitter 99, e.g., WWV, are examples ofsources used by the receiver 93. Such a configuration would be helpfulin the total station system 10 of FIG. 1 in areas with intermittent GPScoverage due to tree canopies or urban-canyon effects. For example,receiver 93 could comprise a commercial product such as is marketed byESE (El Segundo, Calif. 90245), as the ES-180A master clock. The ES-180Areceives and synchronizes to time data broadcast from the NIST viashort-wave radio, WWV in Fort Collins, Colo., and WWVH in Hawaii, andprovides a time-code output (TC89), ASCII time output (queried RS232),and a 1-PPS (pulse-per-second) output.

Time bases that use radio transmissions from the WWV and WWVH stationsoperated by the United States Government typically provide a usablereceived accuracy of one part in ten million for frequency, and aboutone millisecond for timing. The frequencies as transmitted, however, areaccurate to one part in a billion because they are based on the primaryNIST Frequency Standard and related NIST atomic time scales in Boulder,Colo. The difference in transmitted and received accuracy is due tovarious propagation effects.

FIG. 6 is a schematic diagram of a GPS receiver 100 useful in theconfigurations shown in FIGS. 1-3. The GPS receiver 100 incorporates amicroprocessor control unit (MCU) and digital signal processor (DSP)combination 102, e.g., a “SCORPION” integrated circuit designed byTrimble Navigation Limited (Sunnyvale, Calif.). The radio frequencytuning, downconversion, and digital sampling are done with a radiofrequency circuit 104, e.g., a “SURF” integrated circuit designed byTrimble Navigation Limited (Sunnyvale, Calif.). A 10.00 MHz ovenizedcrystal oscillator (OCXO) 106 provides a precision reference frequencyoutput 108 that can be used by the EDM's and total stations described inFIGS. 1-3. Such reference frequency output 108 has very high frequencyprecision, both short term and long term. Signals from orbitingnavigation satellites are used as references and locked on to bytracking loops within the SCORPION 102 and SURF 104 combination. TheMCU/DSP 102 samples the OCXO 106 at its XCLK input and the SURF 104 usesan RO input to generate its local oscillator signals. Alternatively, anexternal 10.00 MHz source maybe connected to input 110. The SCORPION 102and SURF 104 combination computes frequency errors and controls adigital to analog converter (DAC) 111. A DAC output 112 is then able todiscipline the external 10.00 MHz source. A utility 1 PPS output 114 isprovided that can be used as shown in FIGS. 2 and 3.

In FIGS. 1-4, the EDM and GPS oscillator are discussed as beingphysically distinct and separate units. The GPS oscillator is assumed tobe aligned with GPS system time by virtue of its tracking the signals ofthe visible GPS satellites. But in many GPS receivers, e.g., some ofthose marketed by Trimble Navigation (Sunnyvale, Calif.), the GPSreceiver oscillator is not steered or physically aligned with GPS time.Rather, the clock and frequency offsets are calculated and used later in“software” to arrive at accurate solutions. This software technique isextended in embodiments of the present invention to EDM and othersurveying equipment. Such is represented in FIG. 7. As a consequence,such surveying equipment need not be periodically calibrated by astandards laboratory nor certified by government authority. Eachmeasurement in the field is corrected in computer calculations inreal-time to approximately the absolute accuracy of the satellitenavigation system master clocks. The opportunity for long-term drift tocreep in is eliminated as well as the measurement uncertainty that wouldresult.

A system 200 in FIG. 7 is similar to that of FIG. 1, except that theoscillator 43, its control 42, and buffer 41 (all of FIG. 1) are nolonger needed. Also the tracking correction from navigation computer 32to reference oscillator 39 is not used. The navigation computer 32 (232in FIG. 7) computes the clock and frequency offsets which are used laterin “software” to arrive at accurate solutions.

FIG. 7 illustrates a combined satellite positioning and electro-opticaltotal station system embodiment of the present invention, referred toherein by the general reference numeral 200. A global positioning system(GPS) part of the system 200 includes a microwave patch antenna 212 forreceiving L-band transmissions from orbiting GPS satellites, apreamplifier and downconverter 214, a code mixer 216, an in-phasecarrier mixer 218, and a quadrature phase carrier mixer 220. The carriermixer outputs are each sent to low pass filters 222 and 224. Anumerically controlled oscillator (NCO) 226 is driven by a bi-phaselocked loop filter 228 and a multiplier 230. The NCO 226 produces acorrected frequency output that tracks the GPS-satellite carrier beingtracked plus any Doppler effects.

The low pass filter 222 produces a fifty Hertz navigation message thatis input to a navigation computer 232. An adder 234 combines a squaredin-phase signal (I{circumflex over ( )}2) and a squared quadrature-phasesignal (Q{circumflex over ( )}2) to produce a signal-power signal(I{circumflex over ( )}2+Q{circumflex over ( )}2) 236 that isproportional in magnitude to the despreading code correlation. TheI{circumflex over ( )}2+Q{circumflex over ( )}2 signal 236 is used tocontrol the code-phase of a PRN-code generator 238. A GPS-masterreference oscillator 239 provides a precision reference frequency isthen made available to drive an EDM phase and measurement device 244.The GPS code measurement, time measurement, and the navigation messageare used by the navigation computer 232 to compute the currentthree-dimensional position of the system 200. Once the location of theinstrument station is determined either from GPS or other means, aminimum of only one satellite is required to calibrate the time base ofthe instrument.

The electronic distance meter (EDM) part of the system 200 includes theEDM phase measurement device 244 connected to a transmitter 246. Anout-bound signal 247 is directed through a telescope 248 to a distanttarget 250. The target 250 may include a prism corner-cube reflector, oractive repeater for microwave EDM, to return an in-bound signal 251. Thesignals 247 and 251 may be infrared or other laser light, or microwavesignals. The EDM phase measurement subsystem 244 can conduct eitherpulse time-of-flight or carrier phase measurements to determine theline-of-sight distance to the target 250. Conventional methods andequipment can be used to do this. A target range measurement 254 isoutput that can be presented on a local display, recordedelectronically, or transmitted to a user that is at the target and ismoving the target around to mark a particular range from the system 200location.

A theodolite part of the system 200 includes the telescope 248 mountedto an angle measurement instrument 256 connected to a servo actuator258. A theodolite measurement 260 includes an elevation and azimuthoutput that can be presented on a local display, recordedelectronically, or transmitted to a user that is at the target 250 andis moving the target around to mark a particular vector angle from thesystem 200 location. A space vector to target signal 262 is computed bythe navigation computer 232 from a target position seed input 264. Thenavigation computer 232 is able to compute the current position of thesystem 200 and outputs this as a position estimate 266.

The clock and frequency offsets that exist in the hardware are correctedfor in software of navigation computer 232.

From a position estimate, it is possible to determine the altitude andazimuth vector to the target 250. The space vector to target signal 262commands the servo 258 to move the telescope 248 so that it is roughlypointed at the target 250. A conventional search and tracking mechanismcan then be used to find and keep the target 250 visually locked in. Forexample, the Geodimeter SYSTEM-500 is a commercially marketed systemthat is a servo-driven survey instrument in an automatically pointedelectro-optical total station. The target location seed can be computedusing differential satellite position calculations relative to the EDMreference station.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that thedisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A combined satellite positioning andelectro-optical total station system in which the total station isautomatically and precisely calibrated, comprising: a global positioningsystem receiver including a navigation computer configured to receive anavigation message and a PRN-code measurement and to determine carrierphase and frequency of at least one satellite signal, a local referenceoscillator for generating a signal that is referenced to the phase of atleast one cesium-rubidium clock on board a satellite in the GPSsatellite system, said reference oscillator providing a precisionreference frequency and oscillator control to an electronic distancemeasurement system (EDM) to automatically and precisely calibrate saidEDM; an electronic distance measurement (EDM) system with a phasemeasurement subsystem connected to a transmitter for sending anout-bound signal through a telescope to a distant target, and adapted toreceive back an in-bound signal through said telescope, and adapted touse said precision reference frequency to conduct at least one of pulsetime-of-flight and carrier phase measurements to determine aline-of-sight distance to said distant target, wherein said referenceoscillator provides a time base accuracy for said distance measurementcomparable to the accuracy of the cesium-rubidium clock on at least oneGPS satellite.
 2. The system of claim 1, further comprising: atheodolite in which said telescope is mounted to an angle measurementinstrument connected to a servo actuator, and that receives aspace-vector-to-target signal computed by the navigation computer from atarget position seed input, and providing for target signal commands tosaid servo that can move said telescope to be pointed at said distanttarget; wherein, said target location seed is computed usingdifferential satellite position calculations.
 3. A combined satellitepositioning and electro-optical total station system in which the totalstation is automatically and precisely calibrated, comprising: a globalpositioning system receiver including a navigation computer configuredto receive a navigation message and a PRN-code measurement, andproviding a local oscillator signal that is referenced to the frequencyand phase of a cesium-rubidium clock on board a satellite in the GPSsatellite system, the computer further providing a three-dimensionalposition reading of the system; an electronic distance measurement (EDM)phase measurement subsystem connected to the computer to receive saidlocal oscillator signal and automatically and precisely calibrate thetime base of the EDM, the EDM connected to a transmitter for sending anout-bound signal through a telescope to a distant target, and connectedto receive back an in-bound signal through said telescope, and adaptedto use at least one of pulse time-of-flight and carrier phasemeasurements to determine a line-of-sight distance to said target; and atarget range measurement output connected to the EDM phase measurementsubsystem and adapted to provide time-of-flight data information relatedto said line-of-sight distance to said distant target; wherein, thenavigation computer computes the three-dimensional position of thesystem based on said code measurement, said time measurement and saidnavigation message and once said position is determined only onesatellite is required to calibrate the time base of the EDM; and whereinsaid local oscillator signal provides a time base accuracy for said EDMcomparable to the one GPS satellite.
 4. The system of claim 3, furthercomprising: a radio receiver connected to receive a radio transmissionfrom other than a GPS satellite, and which radio transmission includes aprecise and stable transmitted signal that is used to train thereference oscillator during periods the navigation computer is not ableto directly track GPS transmissions.